Low tension surface discharge spark plug



Nov. 13, 1956 w. B. SMlTS LOW 'rsnszou SURFACE DISCHARGE SPARK PLUG Filed June 1, 1953 FIG.

United States Patent LOW TENSION SURFACE DISCHARGE SPARK PLUG Wytze Beye Smits, Voorburg, Netherlands, assignor to Smitsvonk N. V., Leidsehendam, Netherlands, a com- P y Application June 1, 1953, Serial No. 358,845

1 Claim. (Cl. 313-131) The invention relates to a surface discharge spark plug for low voltages, consisting of two concentric electrodes, between which there is disposed a separator body, over the surface of which the spark will creep.

In such spark plugs generally, if the insulating material between the electrodes is soft, it can adjust itself to the expansion and the shrinkage of the inner and outer electrodes. Particularly in motors which grow very hot, so that the sparking surfaces of the plugs also grow very hot, the electrodes will expand greatly and the diameter will increase appreciably, while upon complete cooling these electrodes will shrink again so as to regain their original size. Such a soft mass of material between the electrodes will therefore have to follow these movements, but owing to repeated expansion and shrinkage the mass will soon be mechanically damaged and destroyed owing to the working in the mass itself, especially when the motor is frequently started and stopped.

Now it would be possible to use electrode material and insulating material having the same coeflicient of expansion, but the electrodes have to consist of a material which is highly resistant to elevated temperatures, whereas the consumption of the insulating material over whose surface the spark creeps has to proceed at the same pace as the consumption of the electrode material. The requirements which the two materials have to satisfy are therefore widely diiferent, so that they are hard to reconcile if the temperatures fluctuate in this way. Even if this demand could be complied with, the fact remains that the inner electrode will always grow hotter than the outer electrode, since the latter is cooled by cooling water flowing through the cylinder block. Even if it were possible to achieve a perfect mutual adjustment of the materials for an explosion motor, the temperature fluctuations in explosion motors are still so difierent (as appears from the numerous types of spark plugs for high voltage ignition) that breakage will still occur.

It has been found that the frequency with which the spark plugs heat up and cool down successively tends to shorten their life considerably as compared with the system in which the plug is kept at higher temperatures during a comparatively long time, since crevices are apt to be formed between the electrodes and the insulating material. In fact, it has been found from experiments that when use was made of ceramic material, glass or enamel as insulating material, which had similar coefiicicuts of expansion, small cracks would already appear after 20 to 30 temperature fluctuations in rapid succession; owing to this, the insulating material grew brittle 2,770,750 Patented Nov. 13, 1956 in some places, the discharge spark was formed in the insulating body instead of creeping on the surface, and the spark could not reach the gas mixture any more.

According to the invention a very simple solution for eliminating this drawback has been found, which consists in the spark gap being subdivided in such a way that the insulating body and the electrodes acquire some elasticity in regard to one another without their cohesion being broken. The subdivision may be simple or multiple, and may be effected by grinding or sawing through the insulating body and the electrodes, or by drilling away the insulating material and a small portion of the electrodes in one or more places.

The invention is explained more fully with reference to the drawing illustrating an embodiment.

Fig. 1 is a bottom view of a spark. plug of a known construction. This consists of an outer electrode 1 and an inner electrode 2, between which is disposed the insulating separator body 3.

Fig. 2 is a bottom view of the spark plug according to the invention, in which 1, 2, and 3 are again the outer electrode, the inner electrode, and the insulating separator body respectively. As is evident from this embodiment, the sparking surface is here subdivided, for example, in four places 4, the electrodes 1 and 2 and the insulating body 3 being separated in these four places. As a result of this, the electrodes can expand and shrink freely during the frequent successive stages of heating up and thorough cooling down, without any excessive compressive and tensile force being exerted on the insulating body or the latter splitting or cracking; it has been found that the life of the spark plug is considerably lengthened by this measure.

Fig. 3 illustrates an embodiment in which the insulating material is drilled away, for example, in three places 5, a portion of the electrodes also being removed.

Fig. 4 is a longitudinal section along the line 4-4 of Fig. 2 illustrating the relative disposition of the electrodes, recesses and insulating body.

I claim:

A low voltage surface discharge spark plug comprising an inner electrode having a substantially flat terminal portion, an insulating separator body surrounding said inner electrode and having an end face extending substantially contiguously with the terminal portion of the inner electrode, and an outer electrode disposed concentrically with respect to the inner electrode and the separator body, the outer electrode having a terminal portion extending contiguously with the .terminal portion of the inner electrode, said insulating separator body being in contact with both said terminal portions, spaced recesses formed in the terminal portions of the inner and outer electrodes and in the end face of the separator body, said recesses being in communication with each other and permitting expansion and contraction of the electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 936,507 Anderson Oct. 12, 1909 1,461,405 Solomon July 10, 1923 2,054,134 Mitchell Sept. 15, 1936 2,122,015 St. John June 28, 1938 2,507,278 Smits May 9, 1950 

